Regeneration of bones by the action of osteoblasts, bone formation, is an important phenomenon in vertebrates for maintaining a living body. Factors involved in bone formation include hormones such as estrogen, calcitonin, and parathyroid hormone (PTH); growth factors such as bone morphologenic protein (BMP); and chemicals such as active vitamin D, calcium preparations, and vitamin K2. Estrogen, calcitonin, active vitamin D, and calcium preparations are used as medicine for controlling bone mass in osteoporosis or similar cases. Any of these drugs can be used to prevent decreases of bone mass by inhibiting bone resorption, rather than for increasing bone mass. An effective remedy for enhancing bone formation has thus not been developed yet.
BMP, which is expected to be a new medicine for treating bone disorders, is a unique cytokine that functions as an ectopic formation signal. BMP is thought to effectively form bone (cartilaginous ossification) by replacing cartilaginous callus with new bone cells in repairing fractures or bone deficits (Duprez D. M., Coltey M., Amthor H., Brickell P. M., Tickle, C. (1996) Dev. Biol. 174, 448–452, Bone morphogenetic protein-2.(BMP-2) inhibits muscle development and promotes cartilage formation in chick limb bud cultures; Nakase, T., Nomura, S., Yoshikawa, H., Hashimoto, J., Hirota, S., Kitamura, Y., Oikawa, S., Ono, K., Takaoka, K. (1994) J. Bone Miner. Res. 9, 651–659, Transient and localized expression of bone morphogenetic protein 4 messenger RNA during fracture healing).
There is no conclusive evidence to show that BMP is involved in bone formation caused in conjunction with bone resorption during constant bone formation. Therefore, it is doubtful that BMP can be used as a medicine to promote bone formation by activating and promoting differentiation of osteoblasts that are essential for constant bone formation. Factors involved in constant bone formation have not been reported yet.